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As you can see, the shaft is removible. The problem is that now it's very tight and I can't remove it. I've had this problem several times, but finally I was able to remove it using pincers or squeeze (I'm not sure of the meaning of this words...) But now, I can't remove the shaft even using these tools.

Besides, although I could remove it, next time I use the tuning lever the same problem will appear.

It might just be a very tight fit or some dirt may have gotten around the shaft. I would suggest putting the head into a bench vice for solid support. Put a little bit of penetrating lubricant where the shaft and the collar meet. Heat the locking collar/handle area with a heat gun or hair dryer. When its hot, cool the shaft quickly with some ice cubes, or better yet, the liquid propellant from a can of dust off spray held upside down.

down.http://en.wikipedia.org/wiki/Dust-Off

You can also take a piece of 2X4 lumber a foot long and cut a notch in the end about the width of the shaft. Use this with a hammer as a knock out bar while the head is held in a vice.

Normally, you only remove the tip to exchange it for one of a different size.

Why do you remove the shaft? There is no reason I can see to remove the shaft unless you want to replace it with a longer or shorter one. In any event, once you have the shaft you want on the handle, it stays.

The thread on the shaft is for tool assembly, not for day to day tool use (or playing around).